This spell was made as a way to force conversations.
When the masters of murder realised delayed blast fireball could be used to force conversations, they also realised that it had a way to short duration. So they started trying to make a version that lasts longer. Eventually they did it, but the explosiion was weaker and smaller so they continued until they made the explosion bigger again.
If the entire hour goes by and you havent cancelled the spell you will explode
The explosion will kill the caster. Everybody in ther area of explosion has to make a DEX saving throw, if they fail the caster rolls 3d12 for damage.
If the caster is in possesion of Ajax' Stone Of Magic Protection, they will survive the blast.
To cancel the explosion you have to concetate on canceling it for 1 minute.